PROJECT SUMMARY Diabetes is a devastating disease that is increasing in prevalence. Regular physical activity (PA) markedly improves health for all individuals, including those with diabetes and prediabetes. However, only 5% of American adults are sufficiently active to meet U.S. national guidelines. The health sector is an important setting to promote PA because of its ability to reach and influence large numbers of people. Brief health care provider PA assessment and counseling can successfully increase PA in the short-term (< 6 months), but advice during an outpatient visit is insufficient for lasting PA improvements. We propose a trial including members of Kaiser Permanente Southern California (KPSC), an integrated health plan with high racial/ethnic diversity and whose majority membership is Latino. We will build on an existing KP innovation of assessing PA at every outpatient visit by proposing a trial in which the primary care provider determines the suitability of a patient to increase PA, provides brief advice, and refers patients for PA counseling. Eligible patients are then recruited to a 2-year, medium-intensity telephone behavioral counseling intervention or usual care. We focus the trial on adults who have prediabetes or diabetes who are not prescribed insulin because of their high risk for cardiometabolic complications and the great benefit they can obtain from regular PA. The intervention, individualized by the motivational interviewing approach, is generalizable to other health care delivery systems. Electronic medical records will provide data for secondary outcomes. Our long-term goal is to determine if such a PA intervention can be successfully integrated into primary care services. If so, the intervention can be scaled so millions of sedentary patients with diabetes or prediabetes can receive personal advice and PA counseling. To this end, our primary aim is to determine if the study intervention, compared with usual care, increases moderate-to-vigorous PA (assessed by accelerometry) between baseline and 24 months in patients with type 2 diabetes or prediabetes. Our secondary aims are to 1) determine if the intervention improves body mass index (BMI), hemoglobin A1C, blood pressure, lipids, and quality of life, and 2) determine the cost- effectiveness of the intervention. This study has the potential for a very high impact on clinical preventive practices. PA improves numerous diseases and conditions and is seriously neglected in primary care. By documenting the costs and effects of a practical intervention based in primary care on an objective measure of PA, this study will promote a highly significant shift in primary care practice.